(SPACE.com) -- Those on-the-prowl Mars robots -- Spirit and Opportunity -- are sending back extraordinary images and science data about the Red Planet and its history of climate and water.

Both rovers have found evidence of water at their respective landing sites. But the question remains open as to whether Mars was, or is today, a planet capable of supporting life.

The tell-tale clues of water left behind hint that some spots on Mars did have a persistent wet look that might have been sociable to extraterrestrial creatures. While Mars scientists have their eyes focused on finding tiny microbes, the question remains: just how far along could martian biology, if any, have evolved?

Yet answering this question is a difficult task to answer robotically and it might take rock-splitting fossil hunters, hammer in hand, to chronicle the true life on Mars saga.

Eye of the beholder

Peter Schultz, a planetary geologist at Brown University in Providence, Rhode Island, said you don't have to look out to Mars to see how hard it is to spot fossils.

"We even have trouble identifying fossils on Earth that are older than 3 billion years old. There continues to be debate today," Schultz said. Many primitive life forms leave only subtle traces and often require sophisticated techniques to prove that they indeed were produced by something living, he said.

Those studying the incoming Mars rover images clearly get an eye-full. For many a casual observer, the zones in which the robots tread appear to be chocked full of objects, from weird rocks to leftover remains of long-gone life.

"There are slow geochemical processes that can create spherical shapes and filamentous forms. The 'eye of the beholder' may guide us where to look...but that is not proof," Schultz advised.

Schultz explained that astronomer Percival Lowell, a keen observer who, in the late 1880s into the early 1900s, advocated that Mars was a populated world, once ironically said: "So easily are we the dupes of our own prejudice."

"This certainly is true for what we think we may see in these unprecedented close-up views of Mars [provided by the rovers]. Science requires testing and proving, not simply suggesting," Schultz concluded.

Recognizable patterns

The identification of fossils is often difficult, explained Ron Greeley, Mars Exploration Rover team scientist from Arizona State University in Tempe, even by scientists observing them with the full spectrum of lab instruments.

"Remember that fossils are defined as the traces -- such as leaf imprints in rocks, or the remains, such as shells or bones -- of formerly living organisms. Typically, recognizable patterns are sought, such as bilateral symmetry," Greeley said. "Unfortunately, similar patterns often occur in rocks that result from non-biological processes, which make the identification more difficult."

On Mars, Greeley said, there is no reason to expect the same patterns as fossils that are seen on Earth. Nonetheless, patterns of some sort are being formulated by the astrobiological community -- so-called biomarkers -- in the on-going search for life elsewhere. Furthermore, while the Athena science gear onboard the two Mars rover are great assets, "analyzing patterns and other features remotely is not so easy," he added.

For Greeley, Mars fossil hunting has a bottom line: Unless something really obvious pops out in the images and/or other information, "it's going to take a while to sift through the data and derive some clear answer."

There is a lot of interest in trying to see fossils in the Mars rover images, Greeley said. "The team is looking at the data fairly rigorously, but nothing has emerged along these lines."

Preserved in stone

Prior to 21st century astronauts putting foot and exploration flag down on Mars, there is much that can be done remotely. Future on-the-surface rovers are already being designed, profoundly more capable than the golf cart-sized Spirit and Opportunity.

"Ultimately, it will take sample return of any putative 'fossils' to convince the scientists of the world. But we can tell a lot in situ [on-the-spot] before such a time," said James Garvin NASA Lead Scientist for Mars and Lunar Exploration in the Office of Space Science at the space agency's headquarters in Washington, D.C.

Garvin, a long-time fossil hunter himself, cautioned about the definition of the word. Fossils come in many varieties, Garvin advised, from the micro- and nano-fossils of single-celled primitive microbes, to preserved-in-stone bones of organisms as big as automobiles.

"Searching for fossils also comes in many flavors, from microscopic siftings through tiny grains, to overland reconnaissance for suitable bedding settings to uncover bones," Garvin said.

Microscopic to macroscopic

Assuming that Mars rover data confirms the existence of sedimentary systems of rocks on Mars, searching for fossils can then take many forms, Garvin related. "One would involve the recognition that depositional environments were interrupted and that they are preserved at scales ranging from microscopic to macroscopic."

On Earth, such "preservational" environments are almost always sedimentary, but can include volcanic and impact 'sedimentary' environments.

"So, if the kind of putative sedimentary deposits we have identified on Mars from orbit are validated, then we can gainfully start our first searches for at least indirect, morphological [form and structure of plants and animals] signs of life in such localities," Garvin suggested.

However, expecting fossil formation to operate on Mars the same way as it does on Earth is not a sure bet, Garvin pointed out.

Nooks and crannies

The recent finding by the Opportunity Mars rover of a very high concentration of sulfur in the rock outcrop at Meridiani Planum bodes well for biology and fossil hunting. The chemical form of this sulfur appears to be in magnesium, iron or other sulfate salts

Moreover, Mars is rich in landscape where vestiges of life may be found.

"My overall fossil-hunting bias is heavily weighted to natural caves and fissures and overhangs," said Penelope Boston, Director, Cave and Karst Studies Program at New Mexico Institute of Mining and Technology in Socorro, New Mexico. "I know from personal experience how these environments serve as both original home for organisms and as great places to preserve the evidence," she said.

"I'm particularly excited about the sulfate results...because so much of the material we are looking at is in sulfur rich caves where gypsum and other sulfate minerals play a huge role in the biology and the subsequent preservation of traces of that biology," Boston observed.

Along with caves, Boston said some top-notch nooks and crannies on Earth to look for fossils are in things with vertical relief that slices down through layers, like canyon walls, fault escarpments, and river/stream outflow channels. For Mars and other crater-rich environments, obviously crater walls where things are excavated down to some deeper strata are first-rate sites.

"The best thing about these places is that large -- as in macroscopic -- buried fossilized material is usually weathering out of the outcrops. One can see them laying about and follow the fragments to the source," Boston said. "For microscopic fossils, of course, no clear obvious clues are usually present. The exception is large scale microbial mats like those that made stromatolites and algal travertiles and tufa."

Happy hunting ground

Thanks to picture-taking Mars orbiters, snapshots of Newton Basin on Mars reveal it to be a site were ponded standing water appears to have been present, Boston said. That landscape feature is a great place to look for significant sedimentary rocks, "not to mention Martian critter remains," she added.

"That kind of a no-doubt-about-it pond with minimal shallow excavation could be a goldmine of microfossil material! I'd want to sample its beautiful rims ... and do a small core down through the middle somewhere," Boston explained.

Another promising happy hunting ground for biologists on Mars may be in the cracks within so-called patterned ground or polygonal terrain.

"On Earth, the bottoms of these cracks can house organisms in a much more pleasant environment than the surface at large. Obviously, if such communities thrive or thrived on Mars, some of the evidence may well be at the bottom of these cracks," Boston said. "We have much imaging evidence of these terrains on Mars for comparison to the similar terrains here on Earth, not just at the poles but many other high latitude and high altitude places."

Place your bets

So far there have been no shouts of "eureka!" from Mars rover scientists spotting a signature of past or present life.

"I flat out see no evidence for any fossils in our data," said Jim Rice, a Mars Exploration Rover team scientist and a planetary geologist from Arizona State University in Tempe. "If a trilobite, for example, evolved on Mars and we came across a rock with it, we could resolve it."

Rice noted that the twin Mars rover's each carry a Microscopic Imager. It has a resolution of 30 microns per pixel. That device, however, has the ability to only see objects that are about 100 microns across and above, he added.

"Fossil finding will be a very difficult job on Mars. I don't think robotic missions will do it," Rice said. "It will take astronauts and even then it will be a tough job," he said.

And that job ahead most likely means drilling and cracking a lot of rocks open. On Earth, Rice added, even when you go to a known fossil field location it still requires work to find them.

"We are a lot farther along in the game now that we have identified a rock outcrop on Mars that involved liquid water. We still have much to learn about where to go for any future fossil hunting on Mars. It will be sheer luck if robots discover conclusive evidence for fossils on Mars," Rice said, making the point: "I am putting my bets on astronauts."